Leaf Cutting Bee carrying leaf pieces to build nest

I've read that Leaf Cutting Bees Megachile Latreille mostly construct nests in tubular cavities such as holes drilled into wood, bamboo tubes or hollow stems. But I've never heard of them making nests in spaces between cement bricks!

That is exactly what this Leaf Cutting Bee is doing in our patio.

I noticed last weekend and again this weekend that a bee was always flying into the space between two cement bricks that are part of one of the brick planting bed walls.

This afternoon I decided to sit in a chair to observe exactly what is going on.

As I started to take photos, to my surprise I realized that the bee is always carrying in pieces of leaves. That's when I realized that it is a Leaf Cutting Bee making a nest in the wall space.

This is a very hard working bee.

It brought in a piece of leaf every couple of minutes.

I couldn't see leaf it was cutting up in the garden, but the plant must have been very close.

This is the part I couldn't figure out.

There was another Leaf Cutter Bee that kept hanging out just outside of the nest.

It did fly into the space a couple of times, but then flew right out again.

It almost looked like it was guarding the area.

Here is a fascinating video I found on youtube of a Leaf Cutter Bee creating cup-shaped brood cells with neatly cut pieces of leaves: